Three-piece, wound golf balls with balata covers are preferred by most expert golfers. These balls provide a combination of distance, high spin rate, and control that is not available with other types of golf balls. However, balata is easily damaged in normal play, and, thus, lacks the durability required by the average golfer.
Therefore, most amateur golfers typically prefer a solid, two-piece ball with an ionomer cover, which provides a combination of distance and durability. Because of the hard ionomer cover, these balls are almost impossible to cut, but also have a very hard "feel", which many golfers find unacceptable, and a lower spin rate, making these balls more difficult to draw or fade. The differences in the spin rate can be attributed to the differences in the composition and construction of both the cover and the core.
Recently, multilayer golf balls have become available, in an effort to overcome some of the undesirable aspects of conventional two-piece balls, such as their hard feel, while maintaining the positive attributes of these balls such as their increased initial velocity and distance. Multilayer golf balls typically comprise at least one of multiple core layers, a single or multiple intermediate or mantle layer, and/or multiple cover layers. Ideally, multilayer balls have "feel" and spin characteristics approaching those of wound balls. However, the use of at least one mantle or intermediate layer or of multiple core layers has, in some cases, adversely affected the durability of the core.
A number of elastomers such as polybutadiene, natural rubber, styrene butadiene rubber, and isoprene rubber, have been used in the manufacture of golf ball cores. However, golf ball cores are now predominantly made from compositions comprising polybutadiene. In order to obtain the desired physical properties for golf balls, manufacturers have added cross-linking agents, such as metallic salts of an unsaturated carboxylic acid to the polybutadiene, typically in an amount of about 20 to 50 pph of polybutadiene. Typically, either zinc diacrylate or zinc dimethacrylate is used as the cross-linking agent, with zinc diacrylate providing a higher initial velocity than zinc dimethacrylate.
Typically, about 5 to 50 pph of zinc oxide (ZnO) is also added to the composition, as both a filler and an activation agent for the zinc diacrylate/peroxide cure system. The zinc diacrylate/peroxide cure system, which is well known in the art, cross-links the polybutadiene during the core molding process. In addition to acting as an activation agent, zinc oxide has a high specific gravity (5.6 g/cm.sup.3) that allows the weight of the golf ball to be adjusted.
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with initial velocities in the USGA test that approach the USGA maximum of 250 ft/s plus 2 percent test tolerance or total of 255 ft/s as closely as possible.
To meet the needs of golfers having varying levels of skill and swing speeds, golf ball manufacturers frequently vary the compression of the ball, which is a measurement of the deformation of a golf ball or core in inches under a fixed load. Often, to maximize the initial velocity of a golf ball, the hardness of the core has been increased, which increases the compression of the golf ball. However, in general, the results of durability tests show that hard cores, particularly hard outer cores in dual core golf balls, are less durable than softer cores. Attempts to improve the durability of hard cores by the addition of impact modifiers, such as copolymers of ethylene and glycidyl acrylate, to standard core compositions have resulted in golf balls having a reduced initial velocity, thereby adversely affecting the overall performance of the ball.
Therefore, a need remains for a golf ball core composition containing an impact modifier that provides a golf ball core having improved toughness and impact strength with the initial velocity of prior art golf ball cores that lack an impact modifier. The present invention provides such a core composition for golf balls.